Transmission type liquid crystal display device and the method for fabricating the same

ABSTRACT

The transmission type liquid crystal display device of this invention includes: gate lines; source lines; and switching elements each arranged near a crossing of each gate line and each source line, a gate electrode of each switching element being connected to the gate line, a source electrode of the switching element being connected to the source line, and a drain electrode of the switching element being connected to a pixel electrode for applying a voltage to a liquid crystal layer, wherein an interlayer insulating film formed of an organic film with high transparency and having a plurality of microscopic hollows formed in each pixel is provided above the switching element, the gate line, and the source line, and wherein the pixel electrode formed of a transparent conductive film is provided on the interlayer insulating film.

This application is a division of prior application Ser. No. 08/740,663filed Oct. 31, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transmission type liquid crystaldisplay device which includes switching elements such as thin filmtransistors (hereinafter, referred to as "TFTs") as addressing elementsand is used for displays of computers, TV sets, and the like, and amethod for fabricating such a transmission type liquid crystal displaydevice.

2. Description of the Related Art

FIG. 16 is a circuit diagram of a conventional transmission type liquidcrystal display device provided with an active matrix substrate.

Referring to FIG. 16, the active matrix substrate includes a pluralityof pixel electrodes 1 arranged in a matrix and TFTs 2 used as switchingelements connected to the respective pixel electrodes 1. Gate electrodesof the TFTs 2 are connected to gate lines 3 for supplying a scanning(gate) signal, so that the gate signal can be input into the gateelectrodes to control the driving of the TFTs 2. Source electrodes ofthe TFTs 2 are connected to source lines 4 for supplying an image (data)signal, so that the data signal can be input into the correspondingpixel electrodes 1 via the TFTs when the TFTs are being driven. The gatelines 3 and the source lines 4 run adjacent to the pixel electrodes 1and are arranged in a matrix to cross each other. Drain electrodes ofthe TFTs 2 are connected to the respective pixel electrodes 1 andstorage capacitors 5. Counter electrodes of the storage capacitors 5 areconnected to common lines 6. The storage capacitor 5 is used for holdinga voltage applied to a liquid crystal layer. The storage capacitor isprovided in parallel to a liquid crystal capacitor which includes theliquid crystal layer sandwiched between a pixel electrode provided on anactive matrix substrate and a counter electrode provided on a countersubstrate.

FIG. 17 is a sectional view of a one-TFT portion of the active matrixsubstrate of the conventional liquid crystal display device.

Referring to FIG. 17, a gate electrode 12 connected to the gate line 3shown in FIG. 16 is formed on a transparent insulating substrate 11. Agate insulating film 13 is formed covering the gate electrode 12. Asemiconductor layer 14 is formed on the gate insulating film 13 so as tooverlap the gate electrode 12 via the gate insulating film 13, and achannel protection layer 15 is formed on the center of the semiconductorlayer 14. n⁺ -Si layers as a source electrode 16a and a drain electrode16b are formed covering the end portions of the channel protection layer15 and portions of the semiconductor layer 14, so that they areseparated from each other at the top of the channel protection layer 15.A metal layer 17a which is to be the source line 4 shown in FIG. 16 isformed to overlap the source electrode 16a as one of the n⁺ -Si layers.A metal layer 17b is formed to overlap the drain electrode 16b as theother n⁺ -Si layer so as to connect the drain electrode 16b and thepixel electrode 1. An interlayer insulating film 18 is formed coveringthe TFT 2, the gate line 3, and the source line 4.

A transparent conductive film is formed on the interlayer insulatingfilm 18 to constitute the pixel electrode 1. The transparent conductivefilm is connected to the metal layer 17b which is in contact with thedrain electrode 16b of the TFT 2 via a contact hole 19 formed throughthe interlayer insulating film 18.

Thus, since the interlayer insulating film 18 is formed between thepixel electrode 1 and the underlying layers including the gate andsource lines 3 and 4, it is possible to overlap the pixel electrode 1with the lines 3 and 4. Such a structure is disclosed in JapaneseLaid-Open Patent Publication No. 58-172685, for example. With thisstructure, the aperture ratio improves and, since the electric fieldgenerated by the lines 3 and 4 is shielded, the occurrence ofdisclination can be minimized.

Conventionally, the interlayer insulating film 18 is formed bydepositing an inorganic material such as silicon nitride (SiN) to athickness of about 500 nm by chemical vapor deposition (CVD).

In addition, aside from improvement of the aperture ratio, a prism sheetdescribed on page 217 of Flat-Panel Display 1994 (Nikkei Microdevices,Nikkei Business Publications, Inc.) may be used to enhance thebrightness of the display, or an inserting film described in JapaneseLaid-Open Patent Publication No. 3-141322 may be used to obtain a widerviewing angle.

The above conventional liquid crystal display device has disadvantagesas follows.

When a transparent insulating film made of SiN_(x), SiO₂, TaO_(x), andthe like is formed on the interlayer insulating film 18 by CVD orsputtering, the surface of the film directly reflects the surfaceprofile of the underlying film, i.e., the interlayer insulating film 18.Therefore, when the pixel electrode 1 is formed on the transparentinsulating film, unnecessary steps of approximately 0.1 μm to 1 μm willbe formed on the pixel electrode 1 if the underlying film has steps,causing disturbance in the orientation of liquid crystal molecules.Alternatively, the interlayer insulating film 18 may be formed byapplying an organic material such as polyimide to obtain a flat pixelportion. In such a case, however, in order to form the contact holes forelectrically connecting the pixel electrodes and the drain electrodes, aseries of steps including photo-patterning using a photoresist as amask, etching for forming the contact holes, and removal of thephotoresist are required. A photosensitive polyimide film may be used toshorten the etching and removal steps. In this case, however, theresultant interlayer insulating film 18 appears colored. This is notsuitable for a liquid crystal display device requiring high lighttransmission and transparency.

The other disadvantage is as follows. When the pixel electrode 1overlaps the gate line 3 and the source line 4 via the interlayerinsulating film 18, the capacitances between the pixel electrode 1 andthe gate line 3 and between the pixel electrode 1 and the source line 4increase. In particular, when an inorganic film made of silicon nitrideand the like is used as the interlayer insulating film 18, thedielectric constant of such a material is as high as 8 and, since thefilm is formed by CVD, the thickness of the resultant film is as smallas about 500 nm. With such a thin interlayer insulating film, thecapacitances between the pixel electrode 1 and the lines 3 and 4 arelarge. This causes the following problems (1) and (2). Incidentally, inorder to obtain a thicker inorganic film made of silicon nitride and thelike, an undesirably long time is required in the aspect of thefabrication process.

(1) When the pixel electrode 1 overlaps the source line 4, thecapacitance between the pixel electrode 1 and the source line 4 becomeslarge. This increases the signal transmittance, and thus a data signalheld in the pixel electrode 1 during a holding period fluctuatesdepending on the potential thereof. As a result, the effective voltageapplied to the liquid crystal in the pixel varies, causing, inparticular, vertical crosstalk toward a pixel adjacent in the verticaldirection in the actual display.

In order to reduce the influence of the capacitance between the pixelelectrode 1 and the source line 4 appearing on the display, JapaneseLaid-Open Patent Publication No. 6-230422 proposes a driving methodwhere the polarity of a data signal to be supplied to the pixels isinverted every source line. This driving method is effective for ablack-and-white display panel where the displays (i.e., data signals) ofadjacent pixels are highly correlated with each other. However, it isnot effective for a color display panel for normal notebook typepersonal computers and the like where pixel electrodes are arranged in avertical stripe shape (in color display, a square pixel is divided intothree vertically long rectangular picture elements representing R, G,and B, forming a vertical stripe shape). The display color of pixelsconnected to one source line is different from that of pixels connectedto an adjacent source line. Accordingly, the proposed driving method ofinverting the polarity of the data signal every source line is noteffective in reducing crosstalk for the general color display, though itis effective for the black-and-white display.

(2) When the pixel electrode 1 overlaps the gate line 3 for driving thepixel, the capacitance between the pixel electrode 1 and the gate line 3becomes large, increasing the feedthrough of the write voltage to thepixel due to a switching signal for controlling the TFT 2.

Moreover, such liquid crystal display devices have limits to theirbrightness and viewing angle. In order to improve the brightness and/orthe viewing angle of the liquid crystal display device, theaforementioned prism sheet or inserting film are conventionally used.However, the use of such sheet or film causes the following problems (1)through (6).

(1) Providing an additional prism sheet or inserting film increases thenumber of parts.

(2) Incorporation of an additional prism sheet or inserting film into aliquid crystal display device greatly increases the number offabrication steps.

(3) An additional prism sheet or inserting film increases the thicknessof the liquid crystal display device by approximately several tens ofmicrometers to 1 mm.

(4) Since the prism sheet or the inserting film is incorporated into theliquid crystal display device in later fabrication steps during whichthe degree of air contamination is high, for example, some foreignsubstances could be introduced into the device or surface-originateddamage could occur, thereby greatly lowering the quality of the device.

(5) Incorporation of an additional prism sheet or inserting film intothe liquid crystal device requires extra facilities.

(6) The material used differs depending on what is to be preferentiallyimproved, i.e., either the brightness or the viewing angle. The locationof the prism sheet or the inserting film (i.e., either thebacklight-incident side where the temperature becomes high, or thebacklight-exiting side) is limited depending on the material used. As aresult, the production line changes significantly (for example, theposition of a facility for inserting parts or materials, whether aliquid crystal display device invertor is used or not, etc.) accordingto the kind of device to be fabricated.

SUMMARY OF THE INVENTION

The transmission type liquid crystal display device of this inventionincludes: gate lines; source lines; and switching elements each arrangednear a crossing of each gate line and each source line. A gate electrodeof each switching element is connected to the gate line, a sourceelectrode of the switching element is connected to the source line, anda drain electrode of the switching element is connected to a pixelelectrode for applying a voltage to a liquid crystal layer, wherein aninterlayer insulating film formed of an organic film with hightransparency and having a plurality of microscopic hollows formed ineach pixel is provided above the switching element, the gate line, andthe source line. The pixel electrode, formed of a transparent conductivefilm, is provided on the interlayer insulating film.

In one embodiment of the invention, the plurality of microscopic hollowsfunction as collecting lenses.

In one embodiment of the invention, the plurality of microscopic hollowsform hollows in the pixel electrode for orienting the liquid crystalmolecules in predetermined multiple directions.

In one embodiment of the invention, the device further includes aconnecting electrode for connecting the pixel electrode and the drainelectrode, wherein the interlayer insulating film is provided above theswitching element, the gate line, the source line, and the connectingelectrode. The pixel electrode is formed on the interlayer insulatingfilm so as to overlap at least the gate line or the source line at leastpartially, and the connecting electrode and the pixel electrode areconnected with each other via a contact hole formed through theinterlayer insulating film.

In one embodiment of the invention, the interlayer insulating film ismade of a photosensitive acrylic resin.

In one embodiment of the invention, the interlayer insulating film ismade of a resin which is made transparent by optical or chemicaldecoloring treatment.

In one embodiment of the invention, the pixel electrode and at least oneof the source line and the gate line overlap each other by about 1 μm ormore in a line width direction.

In one embodiment of the invention, the thickness of the interlayerinsulating film is about 1.5 μm or more.

In one embodiment of the invention, the connecting electrode is formedof a transparent conductive film.

In one embodiment of the invention, the device further includes astorage capacitor for holding a voltage applied to the liquid crystallayer, wherein the contact hole is formed above either an electrode ofthe storage capacitor or the gate line.

In one embodiment of the invention, a metal nitride layer is formedbelow the contact hole to connect the connecting electrode and the pixelelectrode.

In one embodiment of the invention, the device further includes astorage capacitor for holding a voltage applied to the liquid crystallayer, wherein a capacitance ratio represented by expression (1):

    Capacitance ratio=C.sub.sd /(C.sub.sd +C.sub.ls +C.sub.s)  (1)

is 10% or less, wherein C_(sd) denotes a capacitance value between thepixel electrode and the source line, C_(ls) denotes a capacitance valueof a liquid crystal portion corresponding to each pixel in anintermediate display state, and C_(s) denotes a capacitance value of thestorage capacitor of each pixel.

In one embodiment of the invention, the shape of the pixel electrode isrectangular with a side parallel to the gate line being longer than aside parallel to the source line.

In one embodiment of the invention, the device further includes adriving circuit for supplying to the source line a data signal of whichpolarity is inverted for every horizontal scanning period, and the datasignal is supplied to the pixel electrode via the switching element.

In one embodiment of the invention, the device further includes astorage capacitor for maintaining a voltage applied to the liquidcrystal layer, the storage capacitor including: a storage capacitorelectrode; a storage capacitor counter electrode; and an insulating filmtherebetween; wherein the storage capacitor electrode is formed in thesame layer as either the source line or the connecting electrode.

In one embodiment of the invention, the storage capacitor counterelectrode is formed of a part of the gate line.

In one embodiment of the invention, the pixel electrode and the storagecapacitor electrode are connected via the contact hole formed above thestorage capacitor electrode.

In one embodiment of the invention, the contact hole is formed aboveeither the storage capacitor counter electrode or the gate line.

In one embodiment of the invention, the interlayer insulating film isformed of a photosensitive resin containing a photosensitive agent whichhas a reactive peak at the i line (365 nm).

According to another aspect of the invention, a method for fabricating atransmission type liquid crystal display device is provided. The methodincludes the steps of: forming a plurality of switching elements in amatrix on a substrate; forming a gate line connected to a gate electrodeof each switching element and a source line connected to a sourceelectrode of the switching element, the gate line and the source linecrossing each other; and forming a connecting electrode formed of atransparent conductive film connected to a source electrode of theswitching element. The method further includes forming an organic filmwith high transparency above the switching elements, the gate lines, thesource lines, and the connecting lines by a coating method andpatterning the organic film to form an interlayer insulating film andcontact holes through the interlayer insulating film to reach theconnecting electrodes, the interlayer insulating film having a pluralityof microscopic hollows formed in each pixel region. The method alsoincludes the step of forming pixel electrodes formed of transparentconductive films on the interlayer insulating film and inside thecontact holes so that each pixel electrode overlaps at least either thegate line or the source line at least partially.

In one embodiment of the invention, the patterning of the organic filmis conducted by either one of the following steps: exposing the organicfilm to light and developing the exposed organic film, or etching theorganic film by using a photoresist on the organic film as an etchingmask.

In one embodiment of the invention, the patterning of the organic filmincludes the steps of: forming a photoresist layer containing silicon onthe organic film; patterning the photoresist layer; and etching theorganic film by using the patterned photoresist layer as an etchingmask.

In one embodiment of the invention, the patterning of the organic filmincludes the steps of: forming a photoresist layer on the organic film;coating a silane coupling agent on the photoresist layer and oxidizingthe coupling agent; patterning the photoresist layer; and etching theorganic film by using the patterned photoresist layer covered with theoxidized coupling agent as an etching mask.

In one embodiment of the invention, the etching step is a step of dryetching using an etching gas containing at least one of CF₄, CF₃ H andSF₆.

In one embodiment of the invention, the organic film is formed by usinga photosensitive transparent acrylic resin which dissolves in adeveloping solution when exposed to light, and the interlayer insulatingfilm and the contact holes are formed by exposing the photosensitivetransparent acrylic resin to light and developing the photosensitivetransparent acrylic resin.

According to another aspect of the present invention, the methodincludes the steps of forming a plurality of switching elements in amatrix on a substrate; forming a gate line connected to a gate electrodeof each switching element and a source line connected to a sourceelectrode of the switching element, the gate line and the source linecrossing each other; forming a connecting electrode formed of atransparent conductive film connected to a source electrode of theswitching element; forming a photosensitive transparent acrylic resinwhich dissolves in a developing solution when exposed to light above theswitching elements, the gate lines, the source lines, and the connectinglines and exposing the photosensitive transparent acrylic resin to lightand developing the photosensitive transparent acrylic resin to form aninterlayer insulating film and contact holes through the interlayerinsulating film to reach the connecting electrodes, the interlayerinsulating film having a plurality of microscopic hollows formed in eachpixel region; and forming pixel electrodes formed of transparentconductive films on the interlayer insulating film and inside thecontact holes so that each pixel electrode overlaps at least either thegate line or the source line at least partially.

In one embodiment of the invention, the method further includes the stepof, after the light exposure and development of the organic film,exposing the entire substrate to light for reacting a photosensitiveagent contained in the photosensitive transparent acrylic resin, therebydecoloring the photosensitive transparent acrylic resin.

In one embodiment of the invention, a base polymer of the photosensitivetransparent acrylic resin includes a copolymer having methacrylic acidand glycidyl methacrylate and the photosensitive transparent acrylicresin contains a quinonediazide positive-type photosensitive agent.

In one embodiment of the invention, the photosensitive transparentacrylic resin for forming the interlayer insulating film has a lighttransmittance of 90% or more for light with a wavelength in the range ofabout 400 nm to about 800 nm.

In one embodiment of the invention, the organic film has a thickness ofabout 1.5 μm or more.

In one embodiment of the invention, the method further includes the stepof, before the formation of the organic film, irradiating withultraviolet light a surface of the substrate where the organic film isto be formed.

In one embodiment of the invention, the method further includes the stepof, before the formation of the organic film, applying a silane couplingagent on a surface of the substrate where the organic film is to beformed.

In one embodiment of the invention, the material for forming the organicfilm contains a silane coupling agent.

In one embodiment of the invention, the silane coupling agent includesat least one of hexamethyl disilazane, dimethyl diethoxy silane, andn-buthyl trimethoxy.

In one embodiment of the invention, the method further includes the stepof, before the formation of the pixel electrode, ashing the surface ofthe interlayer insulating film by an oxygen plasma.

In one embodiment of the invention, the ashing step is conducted afterthe formation of the contact holes.

In one embodiment of the invention, the interlayer insulating filmincludes a thermally curable material and the interlayer insulating filmis cured before the ashing step.

In one embodiment of the invention, the thickness of the ashed portionof the interlayer insulating film is in the range of about 100 to 500nm.

In one embodiment of the invention, the thickness of the pixel electrodeis about 50 nm or more.

In one embodiment of the invention, the interlayer insulating film isformed by developing the photosensitive transparent acrylic resin withtetramethyl ammonium hydroxyoxide developing solution with aconcentration of about 0.1 mol % to about 1.0 mol %.

In one embodiment of the invention, the method further includes the stepof, after the formation of the contact holes through the interlayerinsulating film, decoloring the interlayer insulating film byirradiating the interlayer insulating film with ultra violet light.

In one embodiment of the invention, the method further includes the stepof, before the formation of the organic film, forming a silicon nitridefilm on a surface of the substrate where the organic film is to beformed.

Thus, the invention described herein makes possible the advantage of (1)providing a transmission type liquid crystal display device where flatpixel electrodes overlap respective lines to improve the aperture ratioof the liquid crystal display, minimize disturbance in the orientationof liquid crystal molecules, and simplify the fabrication process.Furthermore, and the influence of the capacitance between the pixelelectrodes and the lines appearing on the display, such as crosstalk,can be reduced to achieve a good display. Moreover, a transmission typeliquid crystal display device is provided which is capable of enhancingthe brightness and the viewing angle without increasing the number ofparts or fabrication steps, providing extra fabrication facilities,increasing the size (thickness, etc.) and changing the fabrication line,while at the same time, not lowering the quality of the device. Theinvention described herein also makes possible the advantage of (2)providing a method for fabricating such a transmission type liquidcrystal display device.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a one-pixel portion of an active matrixsubstrate of a transmission type liquid crystal display device ofExample 1 according to the present invention.

FIG. 2 is a sectional view taken along line A-A' of FIG. 1.

FIG. 3 is a plan view of a one-pixel portion of an active matrixsubstrate of a transmission type liquid crystal display device ofExample 3 according to the present invention.

FIG. 4 is a sectional view taken along line B-B' of FIG. 3.

FIG. 5 is a partial sectional view of an active matrix substrate of atransmission type liquid crystal display device of Example 4 accordingto the present invention.

FIG. 6 is a graph illustrating the relationship between the liquidcrystal charging rate difference and the capacitance ratio fortransmission type liquid crystal display devices of Examples 5 and 6 anda conventional liquid crystal display device.

FIGS. 7A and 7B are waveforms of data signals in the cases of 1Hinversion driving in Examples 5 and 6 and conventional field inversiondriving, respectively.

FIG. 8 is a graph illustrating the relationship between the liquidcrystal capacitance ratio and the overlap width for the transmissiontype liquid crystal display device of Example 5.

FIG. 9 is a plan view of a one-pixel portion of an active matrixsubstrate of a transmission type liquid crystal display device ofExample 7 according to the present invention.

FIG. 10 is a sectional view taken along line C-C' of FIG. 9.

FIG. 11 is a graph illustrating the variation in the transmittancebefore and after light exposure of an acrylic resin, depending on thewavelength (nm) of transmitted light for the transmission type liquidcrystal display device of Example 7.

FIG. 12 is a circuit diagram of a C_(s) -on-gate type liquid crystaldisplay device.

FIG. 13 is a plan view of a one-pixel portion of an active matrixsubstrate obtained by applying the structure of Example 3 to the liquidcrystal display device shown in FIG. 12.

FIG. 14 is a plan view of a one-pixel portion of an active matrixsubstrate of a transmission type liquid crystal display device ofExample 10 according to the present invention.

FIG. 15 is a sectional view taken along line D-D' of FIG. 14.

FIG. 16 is a circuit diagram of a conventional liquid crystal displaydevice provided with an active matrix substrate.

FIG. 17 is a sectional view of a one-pixel portion of the active matrixsubstrate of the conventional liquid crystal display device.

FIGS. 18A, 18B, 18C and 18D are enlarged views each showing a displaysection of a liquid crystal device having microscopic hollows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described by way of examples withreference to the accompanying drawings.

EXAMPLE 1

FIG. 1 is a plan view of a one-pixel portion of an active matrixsubstrate of the transmission type liquid crystal display device ofExample 1 according to the present invention.

Referring to FIG. 1, the active matrix substrate includes a plurality ofpixel electrodes 21 (within a bold line) arranged in a matrix. Gatelines 22 for supplying a scanning (gate) signal and source lines 23 forsupplying an image (data) signal run surround the peripheries of thepixel electrodes 21 and cross each other. The peripheries of each pixelelectrode 21 overlap the gate lines 22 and the source lines 23. A TFT 24acting as a switching element connected to the corresponding pixelelectrode 21 is formed at a crossing of the gate line 22 and the sourceline 23. A gate electrode of the TFT 24 is connected to the gate line 22so that a gate signal can be input into the gate electrode to controlthe driving of the TFT 24. A source electrode of the TFT 24 is connectedto the source line 23 so that a data signal can be input into the sourceelectrode. A drain electrode of the TFT 24 is connected to the pixelelectrode 21 via a connecting electrode 25 and a contact hole 26. Thedrain electrode is also connected to an electrode of a storage capacitor(a storage capacitor electrode 25a) via the connecting electrode 25. Theother electrode of the storage capacitor, a storage capacitor counterelectrode 27, is connected to a common line (element 6 in FIG. 16).Furthermore, a predetermined number (64 in this example) of microscopichollows 28 with a prescribed shape (although circular in this example,the shape is not limited thereto, and may be a polygon, for example) aregenerally formed in an array within a single pixel region of the pixelelectrode 21, the microscopic hollows 28 functioning as a collectivelens. The surface of the hollows are continuous with the surface of thepixel electrode 21. In this example, the diameter of the microscopichollow is around 5 to 15 micrometers.

FIG. 2 is a sectional view of the active matrix substrate taken alongline A-A' of FIG. 1.

Referring to FIG. 2, a gate electrode 32 connected to the gate line 22shown in FIG. 1 is formed on a transparent insulating substrate 31. Agate insulating film 33 is formed covering the gate electrode 32. Asemiconductor layer 34 is formed on the gate insulating film 33 so as tooverlap the gate electrode 32 via the gate insulating film 33, and achannel protection layer 35 is formed on the center of the semiconductorlayer 34. n⁺ -Si layers as a source electrode 36a and a drain electrode36b are formed covering the end portions of the channel protection layer35 and portions of the semiconductor layer 34, so that they areseparated from each other by a portion of the channel protection layer35. A transparent conductive film 37a and a metal layer 37b which are tobe the double-layer source line 23 shown in FIG. 1 are formed to overlapthe source electrode 36a as one of the n⁺ -Si layers. A transparentconductive film 37a' and a metal layer 37b' are formed to overlap thedrain electrode 36b as the other n⁺ -Si layer. The transparentconductive film 37a' extends to connect the drain electrode 36b and thepixel electrode 21 and also serves as the connecting electrode 25 whichis connected to the storage capacitor electrode 25a of the storagecapacitor. An interlayer insulating film 38 is formed covering the TFT24, the gate line 22, the source line 23, and the connecting electrode25.

A transparent conductive film is formed on the interlayer insulatingfilm 38 to constitute the pixel electrode 21 with the contact hole 26and a hollow 28 being formed, the bottom of the hollow 28 having a roundlens-like shape. The pixel electrode 21 is connected to the drainelectrode 36b of the TFT 24 via the contact hole 26 formed through theinterlayer insulating film 38 and the transparent conductive film 37a'which is the connecting electrode 25. Moreover, the plurality ofmicroscopic hollows 28 are formed in the surface of the interlayerinsulating film 38 to a depth of approximately 0.5 to 1.0 micrometer. Analignment film 39 is formed to be flat on the interlayer insulating film38 via the pixel electrode 21. The hollow is formed to have such a depthbecause if the depth of the microscopic hollow 28 is deeper than 1.0micrometer, depending on the material used, the profile of themicroscopic hollows 28 may not be absorbed in the pixel electrode 21 andalignment film 39 of usual thicknesses, making it difficult to form aflat alignment film. The microscopic hollows 28 with a depth of around0.5 to 1.0 micrometer function as collective lenses which collect lightincident from the underlying layers so that the light exits ascollimated beams.

The active matrix substrate with the above structure is fabricated asfollows.

First, the gate electrode 32, the gate insulating film 33, thesemiconductor layer 34, the channel protection layer 35, and the n⁺ -Silayers as the source electrode 36a and the drain electrode 36b aresequentially formed in this order on the transparent insulatingsubstrate 31 such as a glass substrate. This film formation step can beconducted following a conventional method for fabricating an activematrix substrate.

Thereafter, the transparent conductive films 37a and 37a' and the metallayers 37b and 37b' constituting the source line 23 and the connectingelectrode 25 are sequentially formed by sputtering and are patternedinto a predetermined shape.

A photosensitive acrylic resin is applied to the resultant substrate toa thickness of 3 μm, for example, by spin coating to form the interlayerinsulating film 38 made of an organic film with high transparency. Theresultant resin layer is exposed to light so as to irradiate a peripheryof a display region (not shown) and the contact hole 26 for about 5,000milliseconds and the plurality of microscopic hollows for about 100 to500 milliseconds. Thereafter the layer is developed with an alkalinesolution. Only portions of the resin layer exposed to light are etchedwith the alkaline solution, forming the contact holes 26 through theinterlayer insulating film 38 and the microscopic hollows 28 functioningas collective lenses. In one method, a first photomask for the contacthole 26 and the microscopic hollows 28 is used for the time required forthe exposure of the microscopic hollows 28, i.e., 100 to 500milliseconds. Then, for the contact hole 26, a second photomask slightlydifferent from the first photomask in dimension is used for the rest ofthe exposure time. The order of the first and the second masks can bereversed. In such a manner, a smooth contact hole is formed.

Subsequently, a transparent conductive film is formed on the resultantsubstrate by sputtering and is patterned to form the pixel electrodes21. Each pixel electrode 21 is thus connected to the correspondingtransparent conductive film 37a' which is in contact with the drainelectrode 36b of the TFT 24 via the contact hole 26 formed through theinterlayer insulating film 38. In addition, on each pixel electrode 21and the interlayer insulating film 38, the alignment film 39 is formedto have a flat surface above the microscopic hollows 28. In this way,the active matrix substrate of this example is fabricated.

The thus-fabricated active matrix substrate includes the thickinterlayer insulating film 38 between the pixel electrode 21 and theunderlying layers including the gate line 22, the source line 23, andthe TFT 24. With this thick interlayer insulating film, it is possibleto overlap the pixel electrode 21 with the gate and source lines 22 and23 and the TFT 24. Also, the surface of the pixel electrode 21 can bemade flat. As a result, when the transmission type liquid crystaldisplay device including the thus-fabricated active matrix substrate anda counter substrate with a liquid crystal layer therebetween iscompleted, the aperture ratio of this device can be improved. Also,since the electric field generated at the lines 22 and 23 can beshielded, the occurrence of disclination can be minimized. Moreover,with the plurality of microscopic hollows 28 functioning as collectivelenses being provided in each pixel, transmitted light incident from theunderlying layers is collimated to be parallel beams perpendicular tothe substrate 31. As a result, the brightness is increased by 10% ormore compared with that of a device without the microscopic hollows 28.

Since the microscopic hollows 28 functioning as collective lenses areprovided in the interlayer insulating film 38 of the active matrixsubstrate, unlike the use of the conventional prism sheet or insertingfilm, there is no need for increasing the number of parts such as prismsheets or inserting films, the number of steps for fabricating suchparts, or the thickness of the device. The conventional prism sheets orinserting films are fabricated in the later steps of the fabricationduring which the degree of air contamination is high, causing foreignsubstances to be introduced into the product, thus lowering the qualitythereof. The microscopic hollows 28 according to the present inventionare formed in the earlier patterning steps during which the degree ofthe air contamination is low. Therefore, deterioration of the qualitydue to the foreign substances introduced into the product can beprevented without significant modification of the fabrication line.

The acrylic resin constituting the interlayer insulating film 38 has adielectric constant of 3.4 to 3.8 which is lower than that of aninorganic film (e.g., the dielectric constant of silicon nitride is 8)and a high transparency. Also, since the spin coating is employed, athickness as large as 3 μm can be easily obtained. This reduces thecapacitances between the gate line 22 and the pixel electrode 21 andbetween the source lines 23 and the pixel electrodes 21, lowering thetime constant. As a result, the influence of the capacitances betweenthe lines 22 and 23 and the pixel electrode 21 appearing on the display,such as crosstalk, can be reduced, and thus a good and bright displaycan be obtained.

The contact hole 26 can be formed into a sharp tapered shape by thepatterning including the exposure to light and the alkaline development.This facilitates a better connection between the pixel electrode 21 andthe transparent conductive film 37a'.

Further, since the photosensitive acrylic resin is used, the thick filmhaving a thickness of several micrometers can be easily formed by spincoating. No photoresist process is required at the patterning step. Thisis advantageous in production. Though the acrylic resin used as theinterlayer insulating film 38 is colored before the coating, it can bemade transparent optically by exposing the entire surface to light afterthe patterning step. The resin can also be made transparent chemically.

In this example, the photosensitive resin used as the interlayerinsulating film 38 is, in general, exposed to light from a mercury lampincluding the emission spectrum of the i line (wavelength: 365 nm), an hline (wavelength: 405 nm), and a g line (wavelength: 436 nm). The i linehas the highest energy (i.e., the shortest wavelength) among theseemission lines, and therefore it is desirable to use a photosensitiveresin having a reactive peak (i.e., absorption peak) at the i line. Thismakes it possible to form the contact holes with high precision, andmoreover, since the peak is farthest from the visible light, coloringcaused by the photosensitive agent can be minimized. A photosensitiveresin reactive to ultraviolet light having short wavelength emitted froman excimer laser can also be used. By using such an interlayerinsulating film substantially free from coloring, the transmittance ofthe resultant transmission type liquid crystal display device can beincreased. Accordingly, the brightness of the liquid crystal display canbe increased or the power consumption of the liquid crystal display canbe reduced by saving the amount of light needed from a backlight.

Since the thickness of the interlayer insulating film 38 is as large asseveral micrometers, thicker than that in conventional liquid crystaldisplay, a resin with a transmittance as high as possible is preferablyused. The visual sensitivity of a human eye for blue is a little lowerthan those for green and red. Accordingly, even if the spectraltransmittance of the film has slightly lower transmittance for bluelight than that for green and red light, the display quality will be notsubstantially deteriorated. Though the thickness of the interlayerinsulating film 38 was made 3 μm in this example, it is not limited to 3μm. The thickness of the interlayer insulating film may be set dependingon the transmittance and the dielectric constant of the film. In orderto reduce the capacitance, the thickness is preferably equal to orgreater than about 1.5 μm, more preferably equal to or greater thanabout 2.0 μm.

In this example, the transparent conductive film 37a' is formed as theconnecting electrode 25 connecting the drain electrode 36b of each TFT24 and the corresponding pixel electrode 21. This is advantageous in thefollowing points. In the conventional active matrix substrate, theconnecting electrode is composed of a metal layer. When such a metalconnecting electrode is formed in the aperture portion, the apertureratio is lowered. In order to overcome this problem, the connectingelectrode is conventionally formed above the TFT or the drain electrodeof the TFT. The contact hole is formed above the connecting electrodethrough the interlayer insulating film to connect the drain electrode ofthe TFT and the pixel electrode. With this conventional structure,however, when the TFT is made smaller to improve the aperture ratio, forexample, it is not possible to accommodate the entire contact hole abovethe smaller TFT. As a result, the aperture ratio is not improved. Whenthe thickness of the interlayer insulating film is made as large asseveral micrometers, the contact hole should be tapered in order toconnect the pixel electrode and the underlying connecting electrode, anda large-size connecting electrode is required in the TFT region. Forexample, when the diameter of the contact hole is 5 μm, the size of theconnecting electrode should be about 14 μm in consideration of thetapered contact hole and the alignment allowance. In the conventionalactive matrix substrate, if a TFT with a size smaller than this value isrealized, the oversized connecting electrode causes a new problem oflowering the aperture ratio. In contrast, in the active matrix substrateof this example, since the connecting electrode 25 is composed of thetransparent conductive film 37a', no trouble of lowering the apertureratio arises. Further, in this example, the connecting electrode 25extends to connect the drain electrode 36b of the TFT and the storagecapacitor electrode 25a of the storage capacitor formed by thetransparent conductive film 37a'. Since the extension is also formed ofthe transparent conductive film 37a', it does not lower the apertureratio, either.

In this example, the source line 23 is of a double-layer structurecomposed of the transparent conductive layer 37a and the metal layer37b. If part of the metal layer 37b is defective, the source line 23 canremain electrically conductive through the transparent conductive film37a, so that the occurrence of disconnection of the source line 23 canbe reduced.

EXAMPLE 2

In Example 2, another method for forming the interlayer insulating film38 will be described.

The fabrication process until the transparent conductive films 37a and37a' and the metal layers 37b and 37b' are formed by sputtering andpatterned is the same as that described in Example 1. Then, in thisexample, a non-photosensitive organic thin film, is formed on theresultant structure by spin coating. A photoresist is then formed on thethin film and patterned. Using the patterned photoresist, the organicthin film is etched to obtain the interlayer insulating film 38 and thecontact holes 26 formed through the interlayer insulating film 38.Subsequently, formation of a photoresist and patterning are repeatedusing a shorter etching time than the previous etching time to form themicroscopic hollows 28 that do not run through the interlayer insulatingfilm.

Alternatively, the non-photosensitive organic thin film may be formed bya CVD, instead of spin coating.

Examples of the non-photosensitive organic thin film include a thermallycurable acrylic resin. More specifically, JSS-924 (2-component systemacrylic resin) and JSS-925 (1-component system acrylic resin)manufactured by Japan Synthetic Rubber Co., Ltd. can be used. Theseresins generally have a heat resistance of 280° C. or more. Using anon-photosensitive resin for the interlayer insulating film allows forfreer resin design. For example, polyimide resin can be used. Examplesof transparent and colorless polyimide resin include polyimides obtainedby the combination of acid anhydrides such as2,2-bis(dicarboxyphenyl)hexafluoropropylene acid anhydride,oxydiphthalic acid anhydride, and biphenyl tetracaboxylic acidanhydride, with meta-substituted aromatic diamines having a sulfonegroup and/or an ether group or diamines having a hexafluoropropylenegroup. These polyimide resins are disclosed in Fujita, et al., NittoGiho, Vol. 29, No. 1, pp. 20-28 (1991), for example. Among the abovetransparent and colorless polyimide resins, a resin containing both acidanhydride and diamine each having a hexafluoropropylene group has a hightransparency. Fluoric resins other than the above fluoric polyimides canalso be used. Fluoric materials have not only excellent colorlesstransparency but also a low dielectric constant and high heatresistance.

A photoresist containing silicon is preferably used as the photoresistfor the patterning of the interlayer insulating film made of anon-photosensitive organic material. In the above etching, dry etchingis normally conducted using a gas containing CF₄, CF₃ H, SF₆ and thelike. In this case, however, since the photoresist and the interlayerinsulating film are both organic resins, it is difficult to increase theselection ratio between these resins. This is especially true in thecase where the thickness of the interlayer insulating film is as largeas 1.5 μm or more which is nearly the same as that of the photoresist,as in this example. It is preferable that the materials havesufficiently different etching rates (i.e., selectivity). When anacrylic resin is used as the interlayer insulating film in combinationwith a common photoresist material (e.g., OFPR-800 produced by TokyoOhka Kogyo Co., Ltd.) is used, for example, the selection ratio is about1.5. In contrast, in this example, by using the photoresist containingsilicon, a selectivity with respect to the photosensitive acrylic resinof about 2.0 or more can be obtained. Therefore, patterning with highprecision is attained.

Alternatively, at the formation of the interlayer insulating film by thepatterning using a photoresist which does not contain silicon, a silanecoupling agent (e.g., hexamethyl disilazane) may be applied to thephotoresist and the silane coupling agent layer is treated with oxygenplasma to form a silicon oxide film. As a result, the etching rate ofthe photoresist is reduced, since the silicon oxide film on thephotoresist serves as a protection film. This method can be used incombination with the silicon containing photoresist.

The increase in the selection ratio by the above mentioned methodutilizing a silicon element is especially effective in the dry etchingusing a gas containing CF₄, CF₃ H or SF₆.

The active matrix substrate with the thus-formed interlayer insulatingfilm 38 being provided with the microscopic hollows 28 functioning ascollective lenses can also provide a high aperture ratio and a superiorbrightness, as in Example 1.

The non-photosensitive organic thin film used as the interlayerinsulating film 38 in this example has a low dielectric constant and ahigh transparency. The thickness can be as large as 3 μm. With the lowdielectric constant and the long distance between electrodes of thecapacitance, the capacitances between the gate line 22 and the pixelelectrode 21 and between the source line 23 and the pixel electrode 21can be reduced.

EXAMPLE 3

FIG. 3 is a plan view of a one-pixel portion of an active matrixsubstrate of the transmission type liquid crystal display device ofExample 3 according to the present invention. FIG. 4 is a sectional viewtaken along line B-B' of FIG. 3. Components having like functions andeffects are denoted by the same reference numerals as those in FIGS. 1and 2, and the description thereof is omitted.

In the active matrix substrate of this example, each contact hole 26a isformed above the storage capacitor electrode 25a and the storagecapacitor counter electrode 27 of the storage capacitor of each pixel.As described in Example 1, the storage capacitor electrode 25aconstitutes the end portion of the connecting electrode 25 which isconnected to the drain electrode 36b of the TFT 24. The other electrodeof the storage capacitor, the storage capacitor counter electrode 27, isconnected to a counter electrode formed on a counter substrate via thestorage capacitor common line 6 shown in FIG. 16. In other words, thecontact holes 26a are formed above the storage capacitor common line 6which is composed of a light-shading metal film. In addition,rectangular microscopic hollows 29 are formed on which an alignment film39 with step h is formed via a pixel electrode 21.

The above structure of the active matrix substrate of this example hasthe following advantages.

Since the thickness of the interlayer insulating film 38 is as large as3 μm, for example, which is well comparable with the thickness of aliquid crystal cell of 4.5 μm, light leakage tends to occur around thecontact holes 26a due to a disturbance in the orientation of the liquidcrystal molecules. If the contact holes 26a are formed in the apertureportions of the transmission type liquid crystal display device, thecontrast is lowered due to the light leakage. On the other hand, theactive matrix substrate of this example is free from this troublebecause each contact hole 26a is formed above the storage capacitorelectrode 25a and the storage capacitor counter electrode 27 as an endportion of the storage capacitor common line 6 composed of alight-shading metal film. In other words, as long as the contact hole26a is formed above the storage capacitor common line 6 composed of alight-shading metal film, not in the aperture portion, any light leakagewhich may occur around the contact hole 26a due to a disturbance in theorientation of the liquid crystal molecules will not result in loweringof the contrast. This also applies to the case where the storagecapacitor is formed using a portion of the adjacent gate line 22 as oneof electrodes thereof. In this case, the contact hole 26a is formedabove the light-shading gate line 22 and thus lowering of the contrastcan be prevented.

In the active matrix substrate of this example, the connecting electrode25 for connecting the drain electrode 36b of the TFT 24 and the contacthole 26a is composed of the transparent conductive film 37a'.Accordingly, the aperture ratio does not become lower by forming thecontact hole 26a above the storage capacitor.

Thus, in this example, since the storage capacitor counter electrode 27formed under the contact hole 26a shades light, light leakage which mayoccur due to a disturbance in the orientation of the liquid crystalmolecules does not influence the display. The size of the contact hole26a is not necessarily so precise, allowing the hole to be larger andsmoother. As a result, the pixel electrode 21 formed on the interlayerinsulating film 38 is continuous, not being interrupted by the contacthole 26a. This improves the production yield.

Moreover, rectangular microscopic hollows 29 are formed on theinterlayer insulating film 38 to a depth of 1.0 μm to 2.0 μm. A uniformstep h is formed in the surface of the alignment film 39 via pixelelectrode 21 when the rectangular microscopic hollow 29 has the depth of1.0 μm or more. Preferably, the microscopic hollow 29 is formed to havea depth that gives the step h a depth of 50 nm to 0.5 μm (the depth andthe shape of the microscopic hollow 29 do not always agree with those ofthe step of the alignment film 39 depending on the initial viscosity orthe amount of coating). By doing so, the gap between the alignment filmand a counter substrate (not shown) uniformly changes and thussystematically changes the alignment control of the liquid crystaldevice. Accordingly, a desired light-scattering effect is obtainedwidening the viewing angle of the liquid crystal device. Specifically,the microscopic hollow 29 makes a hollow in the alignment film 39 whichorients the liquid crystal molecules in multiple directions and thuswidening the viewing angle.

EXAMPLE 4

FIG. 5 is a partial sectional view of an active matrix substrate of thetransmission type liquid crystal display device of Example 4 accordingto the present invention. Components having like functions and effectsare denoted by the same reference numerals as those in FIGS. 1 to 4, andthe description thereof is omitted.

In the active matrix substrate of this example, each contact hole 26b isformed through the interlayer insulating film 38 above the storagecapacitor common line 6. A metal nitride layer 41, such as a titaniumnitride layer, is formed on the portion of the transparent conductivefilm 37a' under each contact hole 26b.

The above structure of the active matrix substrate of this example isadvantageous in the following point.

Some troubles arise in the adhesion between the resin used for theinterlayer insulating film 38 and ITO (indium tin oxide) used for thetransparent conductive film or metal such as Ta and Al. For example, inthe cleaning process after the formation of the contact hole 26b, acleaning solvent tends to permeate from the contact hole into theinterface between the resin and the underlying transparent conductivefilm, causing the resin film to peel from the transparent conductivefilm. In order to overcome this trouble, according to the active matrixsubstrate of this example, the metal nitride layer 41 made of TaN, AlN,and the like which have good adhesion with the resin is formed on thetransparent conductive film under the contact hole. Accordingly, thepeeling of the resin film and other troubles in the adhesion can beprevented. Furthermore, depth of the contact hole 26b through theinterlayer insulating film 38 becomes shallow as the metal nitride layer41 becomes thick, i.e., the depth of the contact hole 26b is taken up bythe thickness of the metal nitride layer 41. Thus, the dimension of thecontact hole 26b can be made smaller by making the depth of the contacthole 26b shallow.

Any material can be used for the metal nitride layer 41 as long as ithas good adhesion with the resin constituting the interlayer insulatingfilm 38, ITO and the like constituting the transparent conductive film37a', and metal such as Ta and Al. Such a material should also beelectrically conductive to electrically connect the transparentconductive film 37a' and the pixel electrode 21.

EXAMPLE 5

In Example 5, a method for driving the transmission type liquid crystaldisplay device according to the present invention will be described.

In the transmission type liquid crystal display device according to thepresent invention, each pixel electrode overlaps the corresponding linesvia the interlayer insulating film. If the pixel electrode does notoverlap the corresponding lines but gaps are formed therebetween,regions where no electric field is applied are formed in the liquidcrystal layer. This trouble can be avoided by overlapping the pixelelectrode with the lines. The electric field also is not applied to theregions of the liquid crystal layer corresponding to the boundaries ofthe adjacent pixel electrodes. However, light leakage which may occur atthese regions can be blocked by the existence of lines. This eliminatesthe necessity of forming a black mask on a counter substrate inconsideration of an error at the lamination of the active matrixsubstrate and the counter substrate. This improves the aperture ratio.Also, since the electric field generated at the lines can be shielded,disturbances in the orientation of the liquid crystal molecules can beminimized.

The overlap width should be set in consideration of a variation in theactual fabrication process. For example, it is preferably about 1.0 μmor more.

Crosstalk occurs due to the capacitance between the pixel electrode andthe source line when the pixel electrode overlaps the source line asdescribed above. This lowers the display quality of the resultanttransmission type liquid crystal display device. In particular, in aliquid crystal panel used for a notebook type personal computer wherepixels are arranged in a vertical stripe shape, the display is greatlyinfluenced by the capacitance between the pixel electrode and the sourceline. This is considered to be due to the following reasons: (1) Thecapacitance between the pixel electrode and the source line isrelatively large since, in the vertical stripe arrangement, the shape ofthe pixel electrode is rectangular having the side along the source lineas the major side; (2) Since the display color is different betweenadjacent pixels., there is little correlation between signalstransmitted on the adjacent source lines. Thus, the influence of thecapacitance cannot be cancelled between the adjacent source lines.

According to the transmission type liquid crystal display device of thepresent invention, the interlayer insulating film which is composed ofan organic thin film has a small dielectric constant and can be easilythicker. Therefore, the capacitances between the pixel electrodes andthe lines can be reduced, as described above. In addition to thisfeature, according to the method for driving the transmission typeliquid crystal display device of this example, the influence of thecapacitance between the pixel electrode and the source line can bereduced to minimize vertical crosstalk which occurs in notebook typepersonal computers.

The method of this example includes driving the transmission type liquidcrystal display device by inverting the polarity of the data signal forevery horizontal scanning period (hereinafter, this method is referredto as "1H inversion driving").

FIG. 6 shows the influences of the capacitance between the pixelelectrode and the source line upon the charging rate of the pixel in thecases of the 1H inversion driving and a driving method where thepolarity of the data signal is inverted every field (hereinafter, thismethod is referred to as "field inversion driving"). FIGS. 7A and 7Bshow the waveforms obtained by the 1H inversion driving and the fieldinversion driving, respectively.

In FIG. 6, the Y axis represents the charging rate difference whichindicates the ratio of the effective value of the voltage applied to theliquid crystal layer in the gray scale display portion when the grayscale is uniformly displayed to that when a black window pattern isdisplayed in the gray scale display at a vertical occupation of 33%. TheX axis represents the capacitance ratio which is proportional to thevariation in the voltage of the pixel electrode caused by thecapacitance between the pixel electrode and the source line, which isrepresented by expression (1) below:

    Capacitance ratio=C.sub.sd /(C.sub.sd +C.sub.ls +C.sub.s)  (1)

wherein C_(sd) denotes the capacitance value between the pixel electrodeand the source line, C_(ls) denotes the capacitance value of the liquidcrystal portion corresponding to each pixel at the gray scale display,and C_(s) denotes the capacitance value of the storage capacitor of eachpixel. The gray scale display refers to the display obtained when thetransmittance is 50%.

As is observed from FIG. 6, in the 1H inversion driving of this example,the influence of the capacitance between the pixel electrode and thesource line upon the effective voltage actually applied to the liquidcrystal layer can be reduced to one-fifth to one-tenth of that obtainedin the field inversion driving when the capacitance value is the same.This is because, in the 1H inversion driving, the polarity of the datasignal is inverted at intervals sufficiently shorter than the period ofone field during one field. This results in cancelling the influences ofthe positive signal and the negative signal on the display with eachother.

A display test was conducted using a VGA panel with a diagonal of 26 cm.From this test, it was observed that crosstalk was eminent when thecharging rate difference was 0.6% or more, degrading the displayquality. This is shown by the dotted curve in FIG. 6. From the curve inFIG. 6, it is found that the capacitance ratio should be 10% or less inorder to obtain the charging rate difference of 0.6% or less.

FIG. 8 shows the relationships between the overlap amount between thepixel electrode and the source line and the capacitance between thepixel electrode and the source line when the thickness of the interlayerinsulating film is used as a parameter. The VGA panel with a diagonal of26 cm was also used in this test. In the test, the acrylicphotosensitive resin (dielectric constant: 3.4) used in Example 1 wasused as the interlayer insulating film. In consideration of theprocessing precision, the overlap width between the pixel electrode andthe source line should be at least about 1 μm. From FIGS. 6 and 8, it isfound that the thickness of the interlayer insulating film should beabout 2.0 μm or more to satisfy the overlap width of about 1 μm and thecharging rate difference of about 0.6% or less.

Thus, according to the 1H inversion driving of this example, a gooddisplay free from vertical crosstalk can be obtained without invertingthe polarity of the signal on the adjacent source lines (source lineinversion driving) when the pixel electrode overlaps the source line.This method is therefore applicable to notebook type personal computers.

It has also been found that a dot inversion driving has similar effectsto those obtained by the 1H inversion driving. The dot inversion drivingis a driving method where signals of the opposite polarities are inputinto pixel electrodes adjacent each other in the transverse directionand also the polarity is inverted every horizontal scanning period. Asource line inversion driving is also effective when the capacitanceratio is sufficiently low as in the above case. Further, even in thecolor display operation where adjacent signals are not highly correlatedwith each other, colored crosstalk may be suppressed, since thecapacitance between the pixel electrode and the source line issufficiently reduced according to the present invention.

EXAMPLE 6

In Example 6, another method for driving the transmission type liquidcrystal display device according to the present invention will bedescribed. In this method, the polarity of the voltage applied to theliquid crystal layer is inverted for every horizontal scanning period,and simultaneously the signal applied to the counter electrode is drivenby alternate current in synchronization with the inversion of thepolarity of the source signal. This AC driving of the counter electrodecan minimize the amplitude of the source signal.

FIG. 6 described in Example 5 also shows the curve obtained when thecounter electrode is AC driven with an amplitude of 5 V. From FIG. 6, itis observed that, since the 1H inversion driving is employed, thecharging rate difference is sufficiently small compared with the case ofthe field inversion driving, though it is larger by about 10 percentthan that obtained in Example 5 due to the AC driving of the counterelectrode in this example. As a result, a good display without verticalcrosstalk can be realized in the driving method of this example, as inthe previous example.

EXAMPLE 7

FIG. 9 is a plan view of a one-pixel portion of an active matrixsubstrate of the transmission type liquid crystal display device ofExample 7.

In the transmission type liquid crystal display device of this example,each flat pixel electrode overlaps corresponding lines to improve theaperture ratio of the liquid crystal display, minimize disturbances inthe orientation of the liquid crystal molecules, and simplify thefabrication process. Also, the influence of the capacitances between thepixel electrode and the lines appearing on the display, such ascrosstalk, is minimized thereby achieving a good display. In thisexample, an interlayer insulating film with high transparency can beobtained. After the light exposure and development of the interlayerinsulating film, the entire substrate is exposed to light to react theremaining unnecessary photosensitive agent contained in thephotosensitive transparent acrylic resin.

Referring to FIG. 9, the active matrix substrate includes a plurality ofpixel electrodes 51 arranged in a matrix. Gate lines 52 and source lines53 run along the peripheries of the pixel electrodes 51 to cross eachother. The peripheries of each pixel electrode 51 overlap the gate lines52 and the source lines 53. A TFT 54 as a switching element connected tothe corresponding pixel electrode 51 is formed at a crossing of the gateline 52 and the source line 53. A gate electrode of the TFT 54 isconnected to the gate line 52 so that a gate signal is input into thegate electrode to control the driving of the TFT 54. A source electrodeof the TFT 54 is connected to the source line 53 so that a data signalcan be input into the source electrode. A drain electrode of the TFT 54is connected to the corresponding pixel electrode 51 via a connectingelectrode 55 and a contact hole 56. The drain electrode is alsoconnected to an electrode of a storage capacitor, a storage capacitorelectrode 55a, via the connecting electrode 55. The other electrode ofthe storage capacitor, a storage capacitor counter electrode 57, isconnected to a common line. A plurality of microscopic hollows 58functioning as collective lenses are formed in the pixel electrode 51.

FIG. 10 is a sectional view of the active matrix substrate taken alongline C-C' of FIG. 9.

Referring to FIG. 10, a gate electrode 62 connected to the gate line 52shown in FIG. 9 is formed on a transparent insulating substrate 61. Agate insulating film 63 is formed covering the gate electrode 62. Asemiconductor layer 64 is formed on the gate insulating film 63 so as tooverlap the gate electrode 62 via the gate insulating film 63, and achannel protection layer 65 is formed on the center of the semiconductorlayer 64. n⁺ -Si layers as a source electrode 66a and a drain electrode66b are formed covering the end portions of the channel protection layer65 and portions of the semiconductor layer 64, so that they areseparated from each other at the top of the channel protection layer 65.A transparent conductive film 67a and a metal layer 67b which are to bethe double-layer source line 53 shown in FIG. 9 are formed to overlapthe source electrode 66a as one of the n⁺ -Si layers. A transparentconductive film 67a' and a metal layer 67b' are formed to overlap thedrain electrode 66b as the other n⁺ -Si layer. The transparentconductive film 67a' extends to connect the drain electrode 66b and thepixel electrode 51 and also serves as the connecting electrode 55 whichis connected to the storage capacitor electrode 55a. An interlayerinsulating film 68 is formed covering the TFT 54, the gate line 52, thesource line 53, and the connecting electrode 55. The interlayerinsulating film 68 is made of a high-transparency acrylic resin(photosensitive transparent acrylic resin) which dissolves in adeveloping solution when exposed to light.

A transparent conductive film is formed on the interlayer insulatingfilm 68 to constitute the pixel electrode 51. The pixel electrode 51 isconnected to the drain electrode 66b of the TFT 54 via the contact hole56 formed through the interlayer insulating film 68 and the transparentconductive film 67a' which is the connecting electrode 55. Furthermore,the microscopic hollow 58 functioning as collective lenses is formed inthe surface of the interlayer insulating film 68 without running throughthe interlayer insulating film.

The active matrix substrate with the above structure is fabricated asfollows.

First, the gate electrode 62 made of Ta, Al, Mo, W, Cr, and the like,gate insulating film 63 of multiple or single layers made of SiN_(x),SiO₂, Ta₂ O₅, and the like, the semiconductor layer (intrinsic-Si) 64,the channel protection layer 65 made of SiN_(x), Ta₂ O₅, and the like,the n⁺ -Si layers as the source electrode 66a and the drain electrode66b are sequentially formed in this order on the transparent insulatingsubstrate 61 such as a glass substrate.

Thereafter, the transparent conductive films 67a and 67a' and the metallayers 67b and 67b' made of Ta, Al, Mo, W, Cr, and the like constitutingthe source line 53 and the connecting electrode 55 are sequentiallyformed by sputtering and are patterned into a predetermined shape. Inthis example, as in the previous examples, the source line 53 is of thedouble-layer structure composed of the transparent conductive film 67amade of ITO and the metal film 67b. With this structure, if part of themetal layer 67b is defective, the source line 53 can remain electricallyconductive through the transparent conductive film 67a, so that theoccurrences of disconnection of the source line 53 can be reduced.

A photosensitive acrylic resin is applied to the resultant structure toa thickness of 3 μm, for example, by spin coating to form the interlayerinsulating film 68. The resultant resin layer is exposed to lightaccording to a predetermined pattern, then further exposed to light fora short time so as to irradiate the microscopic hollows 58 and the like.The resin layer is thereafter developed with an alkaline solution. Onlythe portions exposed to light are etched with the alkaline solution.Hence, the contact holes 56 through the interlayer insulating film 68and the microscopic hollows 58, and the like, are formed.

Subsequently, a transparent conductive film is formed over theinterlayer insulating film 68 and the contact holes 56 by sputtering andis patterned to form the pixel electrodes 51. Thus, each pixel electrode51 is connected to the transparent conductive film 67a' which is incontact with the drain electrode 66b of the TFT 54 via the contact hole56 formed through the interlayer insulating film 68. In this way, theactive matrix substrate of this example is fabricated.

The interlayer insulating film 68 of Example 7 is made of apositive-type photosensitive acrylic resin, which is a photosensitivetransparent acrylic resin with high transparency which dissolves in adeveloping solution after exposure to light.

The positive-type photosensitive acrylic resin is preferably a materialcomposed of a copolymer of methacrylic acid and glycidyl methacrylate asa base polymer mixed with a naphthoquinone diazide positive-typephotosensitive agent, for example. Since this resin contains theglycidyl group, it can be crosslinked (cured) by heating. After curing,the resin has the properties of: a dielectric constant of about 3.4; anda transmittance of 90% or more for light with a wavelength in the rangeof 400 to 800 nm. The resin can be decolored in a shorter time by beingirradiated with i-line (365 nm) ultraviolet light. Ultraviolet lightother than the i line can be used for patterning. Since the heatresistance of the photosensitive acrylic resin used in this example isgenerally 280° C., the degradation of the interlayer insulating film canbe suppressed by conducting the process such as the formation of thepixel electrodes after the formation of the interlayer insulating filmat a temperature in the range of about 250° C. to 280° C.

The formation of the interlayer insulating film 68 using theabove-described photosensitive acrylic resin with high transparency willbe described in detail.

First, a solution containing the photosensitive transparent acrylicmaterial is applied to the substrate by spin coating, followed by anormal photo-patterning process including prebaking, pattern exposure,alkaline development, and cleaning with pure water in this order.

Specifically, the interlayer insulating film 68 with a thickness of 3 μmis formed by applying a solution containing the photosensitivetransparent acrylic resin to the resultant substrate, preferably to athickness of 4.5 μm, by spin coating. More specifically, the acrylicresin with a viscosity of 29.0 cp is applied at a spin rotation of 900to 1100 rpm. This makes it possible to obtain flat pixel electrodeswithout steps unlike in the conventional method, minimizing disturbancesin the orientation of liquid crystal molecules and improving theresultant display quality.

Subsequently, the resultant substrate is heated to about 100° C. to drya solvent of the photosensitive transparent acrylic resin (e.g., ethyllactate, propylene glycol monomethyl ether acetate, etc.). The resultantphotosensitive acrylic resin is exposed to light twice according to apredetermined pattern and developed with an alkaline solution(tetramethyl ammonium hydroxyoxide, abbreviated to "TMAH"). The portionsof the substrate exposed to light are etched with the alkaline solution,forming the microscopic hollows 58 and the contact holes 56 through theinterlayer insulating film 68. The concentration of the developingsolution is preferably in the range of 0.1 to 1.0 mol % (in the case ofTMAH). When the concentration exceeds 1.0 mol %, the portions of thephotosensitive transparent acrylic resin which are not exposed to lightare also largely etched, making it difficult to control the thickness ofthe photosensitive transparent acrylic resin. When the concentration ofthe developing solution is as high as 2.4 mol %, altered substances fromthe acrylic resin are left in the etched portions as residues, causingcontact failure. When the concentration is less than 0.1 mol %, theconcentration largely varies as the developing solution is circulatedfor repeated use. This makes it difficult to control the concentration.Thereafter, the developing solution left on the substrate surface iswashed away with pure water.

As described above, the interlayer insulating film can be formed by spincoating. Accordingly, the thickness of the film which may be severalmicrometers can be made uniform easily by appropriately selecting therotation of the spin coater and the viscosity of the photosensitivetransparent acrylic resin. The contact hole and the microscopic hollowscan be made into a smooth tapered shape by appropriately selecting theamount of light exposure during the pattern exposure, the concentrationof the developing solution, and the developing time.

The resin may appear colored after the development depending on the typeand amount of the photosensitive agent (e.g., naphthoquinone diazidephotosensitive agents and naphthoquinone diazide positive-typephotosensitive agents) contained in the photosensitive transparentacrylic resin. To avoid this problem, the entire substrate is exposed tolight to allow the remaining unnecessary colored photosensitive agentcontained in the resin to completely react, so as to eliminate lightabsorption in the visible region and thereby to make the acrylic resintransparent. Examples of the photosensitive agent include naphthoxydiazide positive-type photosensitive agents and naphthoquinone diazidephotosensitive agents.

FIG. 11 shows the variation in the light transmittance of the surface ofthe acrylic resin with a thickness of 3 μm before and after beingexposed to light such as ultraviolet light, depending on the wavelength(nm) of the transmitted light. As is observed from FIG. 11, when theresin has not been exposed to light, the transmittance of the resin is65% for transmitted light with a wavelength of 400 nm. After the resinis exposed to light, the transmittance is improved to 90% or more. Inthis case, the substrate was irradiated with light from the front sidethereof. This light exposure step can be shortened by irradiating thesubstrate with light from both the front side and the back side. Thisimproves the throughput of the process.

Finally, the resultant substrate is heated to cure the resin bycrosslinking. More specifically, the substrate is placed on a hot plateor in a clean oven and heated to about 200° C. to cure the resin.

Thus, by using the photosensitive transparent resin, the interlayerinsulating film 68, the microscopic hollows and the contact holes 56formed through the interlayer insulating film 68 for connecting thepixel electrodes and the drain electrodes of the switching elements canbe formed only by conducting the photo-patterning twice without theconventional etching and resist-removing steps. This simplifies thefabrication process. The thickness of the photosensitive transparentacrylic resin may be any desired value in the range of 0.05 to 10 μm (3μm in Example 7; note that the light transmittance lowers and thecoloring is more prominent as the thickness becomes larger) and can bemade uniform by appropriately selecting the viscosity of the resinsolution and the rotation of the spin coater during spin coating.

Thereafter, ITO is deposited on the photosensitive transparent acrylicresin to a thickness of 50 to 150 nm by sputtering and is patterned toform the pixel electrodes 51. The ITO film as each pixel electrode 51having a thickness of 50 nm or more effectively prevents an agent (e.g.,dimethyl sulfoxide) used as a removing solution from permeating fromgaps of the surface of the ITO film into the resin and the resin fromexpanding due to the permeation of the agent. The active matrixsubstrate of Example 7 is thus fabricated.

Thus, in this example, as in the previous examples, with the existenceof the interlayer insulating film 68, all the portions of the displaypanel other than the source and gate line portions can be used as pixelaperture portions. The resultant liquid crystal display device is brightwith high transmittance and a large aperture ratio. In addition, thebrightness is further improved by the formation of the microscopichollows 58 functioning as collective lenses.

Moreover, with the existence of the interlayer insulating film 68, thepixel electrodes can be made flat without being influenced by stepsformed by the underlying lines and switching elements. This prevents theoccurrence of disconnection conventionally found at the steps on thedrain sides of the pixel electrodes, and thereby reduces the number ofdefective pixels. Disturbances in the orientation of liquid crystalmolecules caused by the steps can also be prevented. Furthermore, sincethe source lines 53 and the pixel electrodes 51 are isolated from eachother with the interlayer insulating film 68 therebetween, the number ofdefective pixels conventionally caused by the electrical leakage betweenthe source lines 53 and the pixel electrodes 51 can be reduced.

Further, in this example, the interlayer insulating film 68 can beformed only by the resin formation step, instead of the film formationstep, the pattern formation step with a photoresist, the etching step,the resist removing step, and the cleaning step conventionally required.This simplifies the fabrication process.

In this example, the plurality of microscopic hollows 58 functioning ascollective lenses are provided in the pixel electrode 51. Alternatively,microscopic hollows having light scattering effect can be formed insteadof or with the microscopic hollows 58 by forming steps in the pixelelectrode 51 and further in the alignment film 59 to orient the liquidcrystal molecules in predetermined multiple directions. Althoughcircular microscopic hollows 58 are used in this example, the shape ofthe hollows is not limited thereto. The hollows can be, for example,rectangular or polygonal in shape.

EXAMPLE 8

In Example 8, the method for improving the adhesion between theinterlayer insulating film 68 and the underlying films described inExample 7 shown in FIGS. 9 and 10 will be described.

The adhesion of the photosensitive transparent acrylic resin as theinterlayer insulating film 68 with the underlying films may be inferiordepending on the materials of the underlying films. In such a case,according to the method of this example, the surfaces of the underlyingfilms, i.e., the gate insulating film 63, the channel protection film65, the source electrode 66a, the drain electrode 66b, the transparentconductive films 67a and 67a', and the metal films 67b and 67b' areexposed to ultraviolet light from an M-type mercury lamp (860 W) in anoxygen atmosphere before the application of the photosensitivetransparent acrylic resin, so as to roughen the surfaces. The interlayerinsulating film 68 made of the photosensitive transparent acrylic resinis then formed on the roughened surfaces of the underlying films. Thesubsequent steps are the same as those described in Example 7. By thismethod, the adhesion between the photosensitive transparent acrylicresin and the surface-roughened underlying films improves. Thisovercomes the conventional problem of the film peeling at the interfacebetween the interlayer insulating film 68 made of the photosensitivetransparent acrylic resin and the underlying films. This conditionresults when an agent such as a mixture of hydrochloric acid and ironchloride for etching ITO, permeates into the interface.

Thus, by irradiating the substrate surface before the formation of theinterlayer insulating film 68 with ultraviolet light, the adhesionbetween the interlayer insulating film 68 and the underlying filmsimproves. The resultant device can be stable despite further processingduring the fabrication process.

An alternative method for improving the adhesion according to thepresent invention is to treat the surface to be coated with the resinwith a silane coupling agent before the coating with the resin. As thesilane coupling agent, hexamethyl disilazane, dimethyl diethoxy silane,n-buthyl trimethoxy silane, and the like are especially effective inimprovement of the adhesion. For example, in the case of adhesion withthe silicon nitride film, it has been found that the adhesion strengthof the treated surface improves by about 10% compared with that of thesurface not treated with the silane coupling agent. The problem that thepattern of the resin is damaged due to an internal stress generated bythe crosslinking of the resin, which sometimes occurs if the surface isnot so treated, is prevented by this treatment with the silane couplingagent.

The silane coupling agent may be blended in the resin before theapplication of the resin, instead of applying the agent to theunderlaying layer before the application of the resin. The same adhesioneffect can be obtained by this method. Specifically, when 1 wt % ofdimethyl diethoxy silane was added to the photosensitive acrylic resin,the adhesion strength of the resin with the silicon nitride film (i.e.,a under laying layer) improved by 70%.

EXAMPLE 9

In Example 9, the method for improving the adhesion between theinterlayer insulating film 68 and the pixel electrode material formedthereon described in Example 7 and shown in FIGS. 9 and 10 will bedescribed.

After the formation of the interlayer insulating film 68 made of thephotosensitive transparent acrylic resin in Example 7, the surfaceportion of the interlayer insulating film 68 with a thickness of 100 to500 nm is ashed in an oxygen plasma atmosphere using a dry etchingapparatus. More specifically, the surface of the acrylic resin is ashedin the oxygen plasma atmosphere using a parallel plane type plasmaetching apparatus under the conditions of a RF power of about 1.2 KW, apressure of about 800 m Torr, an oxygen flow rate of about 300 sccm, atemperature of 70° C., and a RF applying time of about 120 seconds. Bythis process, water and carbon dioxide are released from the surface ofthe acrylic resin by oxidation decomposition, and thus the surface isroughened.

Thereafter, ITO is deposited on the roughened photosensitive transparentacrylic resin to a thickness of about 50 to about 150 nm by sputteringand patterned to form the pixel electrodes 51. The active matrixsubstrate is thus fabricated.

By this ashing, the adhesion between the pixel electrodes 51 and theunderlying roughened interlayer insulating film 68 made of thephotosensitive transparent acrylic resin greatly improves. Nodelamination at the interface thereof was caused by an application ofultrasound for cleaning the substrate. The above effect was not obtainedwhen the thickness of the ashed surface portion of the acrylic resin wasless than 100 nm. When it exceeds 500 nm, the decrease in the thicknessof the photosensitive transparent acrylic resin is so large that thevariation in the thickness of the resultant acrylic resin increases,causing display troubles. The improvement in the adhesion is obtained byusing any type of the dry etching apparatus including a barrel type anda RIE type.

Thus, by ashing the surface portion of the interlayer insulating film 68in the oxygen plasma atmosphere before the formation of the pixelelectrodes, the adhesion between the interlayer insulating film 68 andthe pixel electrode material improves. The resultant device can bestable against further processing during the fabrication process. Inaddition, the ashing is also effective in removing residues from thecontact holes. This reduces the occurrence of disconnection in thecontact holes.

In this example, the ashing is conducted after the crosslinking of theresin for the interlayer insulation film. This is advantageous forconducting the ashing step in a more stable condition, since gas isgenerated in the crosslinking step.

EXAMPLE 10

FIG. 14 is a plan view of an active matrix substrate of the transmissiontype liquid crystal display device of Example 10 according to thepresent invention. Microscopic hollows are not shown in the figure. FIG.15 is a sectional view taken along line D-D' of FIG. 14. FIG. 15 alsoshows a counter substrate 42 on which a color filter layer 43 and acounter electrode 44 are disposed. A liquid crystal layer 45 isinterposed between the counter substrate 42 and the active matrixsubstrate. An alignment film formed on the counter electrode is notshown in the figure. Components having like functions and effects aredenoted by the same reference numerals as those in FIGS. 1 and 2, andthe descriptions thereof are omitted.

In the active matrix substrate of this example, the connections betweeneach TFT 24 and the corresponding pixel electrode 21 and between eachstorage capacitor electrode 25a and the corresponding pixel electrode 21are effected via separate contact holes 26a and 26b, respectively. Also,in this example, each source line 23 is composed of a single metallayer, though it may be of a multi-layer structure. The storagecapacitor electrodes 25a are formed of the same material as that of thesource lines 23 in the same step as in the previous examples. The twocontact holes 26a and 26b are formed above a metal electrode 23bconnected to the drain electrode 36b of the TFT and above the storagecapacitor electrode 25a, respectively. That is, these contact holes 26aand 26b formed above the metal electrodes having a light-shadingproperty.

The transmission type liquid crystal display device with the abovestructure is advantageous in the following points.

When the thickness of the interlayer insulating film 38 is as large as 3μm, for example, which is well comparable with the typical thickness ofa liquid crystal layer (a cell thickness) of 4.5 μm, light leakage tendsto occur around the contact holes 26a and 26b due to disturbances in theorientation of the liquid crystal molecules. If the contact holes 26aand 26b are formed in the aperture portions of the transmission typeliquid crystal display device, the contrast ratio is lowered due to thelight leakage. In contrast, the active matrix substrate of this exampleis free from this trouble because the storage capacitor electrode 25ablocks the light from around the contact holes 26b and the metalelectrode 23b blocks the light from around the contact holes 26a. Theaperture ratio can be further increased by forming the storage capacitorcounter electrodes 27 so that they do not extend from the storagecapacitance electrode 25a. Though the C_(s) -Common type was used inthis example, the C_(s) -on-Gate type can also be used.

Thus, in Examples 1 to 10 above, each pixel electrode overlaps thecorresponding lines to improve the aperture ratio of the liquid crystaldisplay, to minimize disturbances in the orientation of the liquidcrystal molecules, and to simplify the fabrication process. Also, theinfluence of the capacitances between the pixel electrode and the linesappearing on the display, such as crosstalk, is minimized to achieve agood display. In addition to these features, a wide viewing angle can beobtained. Moreover, by using microscopic hollows functioning ascollective lenses or microscopic hollows having a light-scatteringeffect by use of steps, the brightness and/or the viewing angle can beenhanced.

The wide viewing angle can be obtained due to the following reasons: (1)The orientation of the liquid crystal molecules is not disturbed sincethe surfaces of the pixel electrodes are flat; (2) No disclination lineis generated due to the electric field generated at the lines; (3)oblique light from the backlight can be effectively used by having theinterlayer insulating film 38 as thick as several micrometers while thedistance between adjacent aperture portions is in the range of severalmicrons to ten and several microns; and (4) The contrast is large (1:300or more for a 10.4-inch SVGA). As a result, the retardation value, i.e.,the refractive index anisotropy of liquid crystal (Δn)×cell thickness(d), can be reduced. This reduction of the retardation is obtainedmainly by reducing the cell thickness according to the presentinvention. In general, as the value of Δn×d decreases, the viewing angleincreases but the contrast decreases. According to the presentinvention, however, the size of the pixel electrodes is made large byeliminating the margins conventionally provided between the pixelelectrodes and the corresponding lines. For example, for a 10.4" VGA,the aperture ratio increased by about 20 points from 65% to 85%, and thebrightness also increased by more than 1.5 times. For a 12.1" XGA,similarly, the aperture ratio greatly increased from 55% to 80%. Thereason is as follows. In the conventional structure, when the sourceline width is 6 μm, the gap between the source line and the pixelelectrode is 3 μm, and the attachment margin is 5 μm, for example, thedistance between adjacent aperture portion is required to be 22 μm ormore. In contrast, according to the present invention where each pixelelectrode overlaps the corresponding source lines, the distance betweenadjacent aperture portions can be 6 μm which is the source line width.Thus, the ratio of the portion which does not constitute the apertureportion to the entire area can be greatly reduced. Hence, since thecontrast is improved, although the value of Δn×d is reduced to widen theviewing angle, the contrast is not lowered. This is eminent in the caseof a TN type liquid crystal device in particular.

Examples 3 and 4 described the transmission type liquid crystal displaydevice where one electrode of the storage capacitor (a storage capacitorelectrode) is connected to the counter electrode via the storagecapacitor common line. The same effects obtained by the above structurecan also be obtained by using the gate line 22 of the adjacent pixel asthe storage capacitor electrode. FIGS. 12 and 13 show the latterstructure (with no microscopic hollows being shown). This type of liquidcrystal display device is called a C_(s) -on-gate type, where each pixelelectrode 21 overlaps the immediately before or next gate line 22 toform a storage capacitor C_(s). In this case, the pixel electrode 21preferably overlaps a larger portion of the immediately before or nextgate line while it overlaps a smaller portion of the corresponding gateline. In this case also, the microscopic hollows functioning ascollective lenses may be formed in the pixel electrode 21 to orient theliquid crystal molecules in multiple directions.

In Examples 1 to 10, the photosensitive transparent acrylic resin withhigh transparency is applied by spin coating and patterned to form theinterlayer insulating film, and the contact holes are formed through theinterlayer insulating film. The application of the photosensitivetransparent acrylic resin can also be conducted by methods other thanthe spin coating, such as roll coating and slot coating. The effects ofthe present invention can also be obtained by these methods. Rollcoating is a method where a substrate is allowed to pass through betweena roll with an uneven surface and a belt with the surface of thesubstrate to be subjected to the coating facing the roll. The thicknessof the resultant coating is determined by the degree of the unevenness.The slot coating is a method where a substrate is allowed to pass underan ejection slot. The thickness of the resultant coating is determinedby the width of the ejection slot.

In Examples 7 and 8, among the i line (wavelength: 365 nm), the h line(wavelength: 405 nm), and the g line (wavelength: 436 nm) generally usedfor the light exposure process, the i line having the shortestwavelength is used. This shortens the light irradiation time, and ishighly effective in decoloring in Example 7 and in roughening thesurface in Example 8.

Furthermore, FIGS. 18A, 18B, 18C and 18D are views showing results ofexperiments seen by microscope when the number of the above-describedmicroscopic hollows is set to, for example, about 20 to 240 per pixel.In the figures, the number of the microscopic hollows (shown by hatchedcircles in the figures) differs at every two rows of pixels. The numberof microscopic hollows in one pixel is not limited the above examplesand may be set depending on required characteristics (i.e., brightness,viewing angle range or the like) of liquid crystal display devices.

Thus, according to the present invention, with the existence of theinterlayer insulating film, each pixel electrode can be formed tooverlap the corresponding lines. This improves the aperture ratio andminimizes disturbances in the orientation of the liquid crystalmolecules. Since the interlayer insulating film is composed of anorganic thin film, the dielectric constant thereof is smaller and thethickness thereof can be easily larger, compared with an inorganic thinfilm. Thus, the capacitances between the pixel electrode and the linescan be reduced. As a result, vertical crosstalk caused by thecapacitance between the pixel electrode and the source line can bereduced, and the feedthrough of the write voltage to the pixels causedby the capacitance between the pixel electrode and the gate line, aswell as the variation in the fabrication process, can be reduced.

In the formation of the interlayer insulating film, the photosensitiveorganic material such as an acrylic resin is applied to the substrate bya coating method and patterned by light exposure and development toobtain an organic thin film with a thickness of several micrometers withhigh productivity. This makes it possible to fabricate the transmissiontype liquid crystal display device with a high aperture ratio withoutlargely increasing production cost. The transmission type liquid crystaldisplay device with a high aperture ratio can also be obtained byforming the organic thin film by deposition, forming a photoresist onthe organic thin film, and patterning the organic thin film in anetching process. In the case where the resin used for the interlayerinsulating film is colored, the resin can be made transparent byoptically or chemically decoloring the resin after the patterning. As aresult, a good color display can be obtained.

The connecting electrode for connecting the drain electrode of the TFTand the pixel electrode is formed using the transparent conductive film.This further improves the aperture ratio. The transparent conductivefilm can be formed simultaneously with the source line which is of adouble-layer structure including the transparent conductive film. Withthe double-layer structure, disconnection at the source line can beprevented.

Each contact hole is formed through the interlayer insulating film abovethe storage capacitor common line or the gate line (i.e., scanningline). This improves the contrast ratio because light leakage which maybe generated due to a disturbance in the orientation of the liquidcrystal can be blocked by the storage capacitor portion. In other words,light leakage is generated in the light-shading portions, if generated,not in the aperture portions.

The metal nitride layer may be formed under each contact hole formedthrough the interlayer insulating film. This improves the adhesionbetween the interlayer insulating film and the underlying film. Thus,the resultant liquid crystal display device is stable against furtherprocessing in the production process.

Each pixel electrode may overlap the corresponding source line by about1 μm or more. With this overlap, the aperture ratio can be maximized.Also, the processing precision of each pixel electrode with respect tothe corresponding lines is not necessarily required to be high. This isbecause, even if the processing precision is low, light leakage can bewell blocked by the overlapping lines as long as the pixel electrodeoverlaps the lines.

By having the thickness of the interlayer insulating film be 1.5 μm ormore (preferably, 2.0 μm or more), the capacitance between each pixelelectrode and the corresponding source line can be sufficiently small.This reduces the time constant even though the pixel electrode overlapsthe source line by about 1 μm or more. As a result, the influence of thecapacitance appearing on the display, such as crosstalk, can be reduced,and thus a good display can be provided.

The vertical crosstalk is further reduced by decreasing the capacitanceratio represented by expression (1) above to 10% or less, since thecapacitance between the pixel electrode and the source line issufficiently reduced.

The polarity of the data signal supplied from the source line may beinverted every gate line. This further reduces the influence of thecapacitance between each pixel electrode and the corresponding sourceline appearing on the display, such as vertical crosstalk.

The effects of the present invention can also be obtained for the activematrix substrate where the pixel electrodes are arranged in a verticalstripe shape and each pixel electrode is of a rectangular shape with theside thereof parallel to the source line being longer than the sidethereof parallel to the gate line. This makes it possible to obtain alarge-scale liquid crystal display device with a high aperture ratiofree from vertical crosstalk for notebook type personal computers andthe like.

Each storage capacitor is formed using an insulating film which isextremely thinner than the interlayer insulating film. The resultantstorage capacitor can have a large capacitance while the area thereof issmall. This improves the aperture ratio. Since the storage capacitorelectrodes are formed simultaneously with the source lines (i.e., signallines), an increase in the number of process steps can be avoided.

When the source lines are composed of lightshading conductive films, thecontact hole portions can be blocked from light. This concealsdisturbances in the orientation of the liquid crystals occurring atthese portions, improving the display quality. This also improves theaperture ratio.

In the case of using a photosensitive resin reactive to ultravioletlight, if the resin has a reactive peak at the i line, the contact holescan be formed with high precision. Also, since the peak is farthest fromthe visible light, coloring can be minimized. This improves thetransmittance of the resultant transmission type liquid crystal displaydevice, and thus the amount of light from a backlight can be reduced,saving power consumption, or the brightness can be increased if theamount of light from the backlight is not reduced.

Since the interlayer insulating film according to the present inventionis comparatively thick and can be made flat, conventional troublescaused by steps formed by the underlying lines and the like, such asdisconnection on the drain side of the pixel electrode, are overcome.However, by forming steps precisely and uniformly, a significantly wideviewing angle can be obtained. Disturbances in the orientation of theliquid crystal is also prevented. The pixel electrodes and the lines areisolated by the interlayer insulating film formed therebetween. Thisgreatly reduces the number of defective pixels due to electrical leakagebetween the pixel electrodes and the lines, thereby increasingproduction yield and reducing production cost. Moreover, according tothe present invention, the interlayer insulating film can be formed onlyby the resin formation step, instead of the film formation step, thepattern formation step with a photoresist, the etching step, the resistremoving step, and the cleaning step conventionally required. Thissimplifies the fabrication process and reduces production cost.

Besides, the microscopic hollows are formed in the interlayer insulatingfilm for collecting or scattering light, thereby improving thebrightness and/or the viewing angle. As a result, a liquid crystaldisplay device with wider viewing angle and/or higher brightnessaccording to different usages can be easily fabricated with highproduction yield and less installation cost without increasing thenumber of parts, fabrication steps, thickness or the like.

The entire substrate may be exposed to light to allow the remainingunnecessary photosensitive agent contained in the photosensitivetransparent acrylic resin to completely react after the light exposureand development of the interlayer insulating film. With this process, aninterlayer insulating film with higher transparency can be obtained.

The surface of the substrate before the formation of the interlayerinsulating film may be irradiated with ultraviolet light. This improvesthe adhesion between the interlayer insulating film and the underlyingfilm. Thus, the resultant liquid crystal display device can be stableagainst further processing in the production process.

The surface of the interlayer insulating film may be ashed in an oxygenplasma atmosphere before the formation of the film of pixel electrodematerial. This improves the adhesion of the interlayer insulating filmand the film of the pixel electrode material formed thereon. Thus, theresultant liquid crystal display device can be stable against furtherprocessing in the production process.

The pixel electrodes with a thickness of 50 nm or more can effectivelyprevent an agent used as a removing solution from permeating from gapsof the film surface into the resin and the resin from expanding due tothe permeation of the agent.

The light irradiation time can be shortened and the decoloringefficiency is high by using the i line (wavelength: 365 nm) havinghigher energy than visible light.

As the aperture ratio of the display improves, the brightness thereofalso improves. Accordingly, the viewing angle can be widened by reducingthe retardation without degrading the contrast. This makes it possibleto obtain a significantly wide viewing angle. Additionally, theplurality of microscopic hollows formed in the pixel further enhancesthe brightness and/or the viewing angle.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A method for fabricating a transmission typeliquid crystal display device, comprising the steps of:forming aplurality of switching elements in a matrix on a substrate; forming agate line connected to a gate electrode of each switching element and asource line connected to a source electrode of the switching element,the gate line and the source line crossing each other; forming aconnecting electrode formed of a transparent conductive film connectedto a source electrode of the switching element; forming an organic filmwith high transparency above the switching elements, the gate lines, thesource lines, and connecting lines by a coating method and patterningthe organic film to form an interlayer insulating film and contact holesthrough the interlayer insulating film to reach the connectingelectrodes, the interlayer insulating film having a plurality ofmicroscopic hollows formed in each pixel region; and forming pixelelectrodes formed of transparent conductive films on the interlayerinsulating film and inside the contact holes so that each pixelelectrode overlaps at least either the gate line or the source line atleast partially.
 2. A method according to claim 1, wherein thepatterning of the organic film is conducted by either one of thefollowing steps:exposing the organic film to light and developing theexposed organic film, or etching the organic film by using a photoresiston the organic film as an etching mask.
 3. A method according to claim2, wherein the patterning of the organic film includes the stepsof:forming a photoresist layer on the organic film; coating a silanecoupling agent on the photoresist layer and oxidizing the couplingagent; patterning the photoresist layer; and etching the organic film byusing the patterned photoresist layer covered with the oxidized couplingagent as an etching mask.
 4. A method according to claim 2, wherein thepatterning of the organic film includes the steps of:forming aphotoresist layer containing silicon on the organic film; patterning thephotoresist layer; and etching the organic film by using the patternedphotoresist layer as an etching mask.
 5. A method according to claim 4,wherein the etching step is a step of dry etching using an etching gascontaining at least one of CF₄, CF₃ H and SF₆.
 6. A method according toclaim 1, wherein the organic film is formed by using a photosensitivetransparent acrylic resin which dissolves in a developing solution whenexposed to light, and the interlayer insulating film and the contactholes are formed by exposing the photosensitive transparent acrylicresin to light and developing the photosensitive transparent acrylicresin.
 7. A method according to claim 6, further including the step of,after the formation of the contact holes through the interlayerinsulating film, decoloring the interlayer insulating film byirradiating the interlayer insulating film with ultraviolet light.
 8. Amethod according to claim 1, further including the step of, after thelight exposure and development of the interlayer insulating film,exposing the entire substrate to light for reacting a photosensitiveagent contained in the photosensitive transparent acrylic resin, therebydecoloring the photosensitive transparent acrylic resin.
 9. A methodaccording to claim 1, further including the step of, before theformation of the organic film, applying a silane coupling agent on asurface of the substrate where the organic film is to be formed.
 10. Amethod according to claim 9, wherein the silane coupling agent includesat least one of hexamethyl disilazane, dimethyl diethoxy silane, andn-buthyl trimethoxy.
 11. A method according to claim 1, wherein thematerial for forming the organic film contains a silane coupling agent.12. A method according to claim 1, further including the step of, beforethe formation of the pixel electrode, ashing the surface of theinterlayer insulating film by an oxygen plasma.
 13. A method accordingto claim 12, wherein the ashing step is conducted after the formation ofthe contact holes.
 14. A method according to claim 12, wherein theinterlayer insulating film includes a thermally curable material and theinterlayer insulating film is cured before the ashing step.
 15. A methodaccording to claim 12, wherein the thickness of the ashed portion of theinterlayer insulating film is in the range of about 100 to 500 nm.
 16. Amethod according to claim 1, wherein the thickness of the pixelelectrode is about 50 nm or more.
 17. A method according to claim 1,further including the step of, before the formation of the organic film,forming a silicon nitride film on a surface of the substrate where theorganic film is to be formed.
 18. A method for fabricating atransmission type liquid crystal display device, comprising the stepsof:forming a plurality of switching elements in a matrix on a substrate;forming a gate line connected to a gate electrode of each switchingelement and a source line connected to a source electrode of theswitching element, the gate line and the source line crossing eachother; forming a connecting electrode formed of a transparent conductivefilm connected to a source electrode of the switching element; forming aphotosensitive transparent acrylic resin which dissolves in a developingsolution when exposed to light above the switching elements, the gatelines, the source lines, and connecting lines and exposing thephotosensitive transparent acrylic resin to light and developing thephotosensitive transparent acrylic resin to form an interlayerinsulating film and contact holes through the interlayer insulating filmto reach the connecting electrodes, the interlayer insulating filmhaving a plurality of microscopic hollows formed in each pixel region;and forming pixel electrodes formed of transparent conductive films onthe interlayer insulating film and inside the contact holes so that eachpixel electrode overlaps at least either the gate line or the sourceline at least partially.
 19. A method according to claim 18, wherein abase polymer of the photosensitive transparent acrylic resin includes acopolymer having methacrylic acid and glycidyl methacrylate and thephotosensitive transparent acrylic resin contains a quinonediazidepositive-type photosensitive agent.
 20. A method according to claim 19,wherein the interlayer insulating film is formed by developing thephotosensitive transparent acrylic resin with tetramethyl ammoniumhydroxyoxide developing solution with a concentration of about 0.1 mol %to about 1.0 mol %.
 21. A method according to claim 18, wherein thephotosensitive transparent acrylic resin for forming the interlayerinsulating film has a light transmittance of 90% or more for light witha wavelength in the range of about 400 nm to about 800 nm.
 22. A methodaccording to claim 18, wherein the photosensitive transparent acrylicresin has a thickness of about 1.5 μm or more.
 23. A method according toclaim 18, further including the step of, before the formation of thephotosensitive transparent acrylic resin, irradiating with ultravioletlight a surface of the substrate where the organic film is to be formed.